1. Field of the Invention
The present invention pertains to a method and apparatus for minimizing the generation and transmission of acoustic vibrations in fluid transmission components such as piping systems and pressure vessels. The method and apparatus minimize the transmission and effect of fluid pressure pulsations at resonant frequencies of the transmission components and due to the velocity modulations of the fluid flow entering and leaving a component.
2. Background
The elimination of acoustic vibrations or pressure pulsations in fluid transmission systems and the adverse affects thereof has been a longstanding problem in the design of various types of fluid transmission equipment including compressed air networks, gas process and transmission systems, internal combustion engine exhaust systems, and other types of fluid processing and transmission apparatus. A widely accepted method of dampening the pressure pulsations generated by positive displacement air and gas compressors, pumps and other fluid handling devices and the muffling of internal combustion engine exhaust noise comprises the provision of a combination of conduits and somewhat enlarged pressure vessels interposed in the conduits to dampen or suppress acoustic pulsations and the mechanical vibrations induced by such pulsations in the conduit system.
In accordance with the teaching of U.S. Pat. Nos. 2,936,041 to J. M. Sharp, et al and 2,997,124 to G. Damewood, et al, it has been determined that so-called acoustic filters or pulsation suppressing devices can be designed for use in fluid transmission systems to have a cutoff frequency below the lowest fundamental frequency generated by a compressor, pump or other device and to dampen certain pressure waves of different frequencies. Moreover, the teachings of the Sharp, et al and Damewood, et al patents also are directed to suppressing the transmission of pressure pulsations and pulsation induced mechanical vibrations of acoustic filter network components by locating points of communication of the fluid inlet and outlet conduits with a pressure vessel at a point along the vessel length corresponding to a pressure node of the standing pressure wave of the frequency to be suppressed.
In particular, the prior art suggests the provision of an acoustic filter in which the transmission of resonant frequencies between the filter and piping elements is minimized through suitable locations of the flow junctures of the inlets and outlets of various enlarged pressure vessels also termed acoustical capacitances. The Sharp, et al and Damewood, et al patents suggest that the acoustic vibrations generated in an acoustic capacitance comprising a pressure vessel, frequently also termed a bottle, manifold or header, can be minimized by connecting the fluid flow inlet conduits to the vessel at the midpoint of the so-called acoustic length of the vessel so as to suppress pulsations of a frequency corresponding to the fundamental vessel length resonant frequency and odd harmonics of the length resonant frequency. Since standing pressure waves of even harmonic frequencies have pressure maxima occurring simultaneously at opposite closed ends of an elongated generally uniform diameter pressure vessel type acoustic capacitance and the wave pressure minima also occur simultaneously, if the areas at the ends of the vessel are equal the mechanical forces generated by the even harmonic standing waves at opposite ends of the vessel are balanced and mechanical vibrations do not occur. However, the odd harmonic standing waves have pressure maxima occurring at one end of the vessel while at the same time pressure minima occur at the other end. This, of course, causes unbalanced forces to develop which result in mechanical vibration. However, with the introduction of fluid to an acoustic filter or capacitance of the type described at a pressure node of the resonant frequency standing pressure wave, or standing pressure waves of odd harmonics of the resonant frequency, incoming pulsations of the same frequency will be effectively cancelled. Moreover, resonant vibrations in the vessel connot be excited by the in feeding of pressure pulsations at such a node.
If pressure pulsations already exist in a pressure vessel at one of its resonant frequencies then a conduit opening into the vessel located at a pressure node will not be subjected to pulsating flow at the frequency of the corresponding standing wave. It has been suggested that this is particularly true as long as the intersection of the pressure vessel with the conduit in question has no internal protrusions and as long as the diameter of the conduit is relatively small as compared with the wavelength of the resonant pulsations.
The Damewood, et al patent, also, suggests that the inlet flow conduits into an elongated pressure vessel type acoustic capacitance should extend substantially parallel to the longitudinal axis of the vessel and open into the vessel in a direction along its axis. On the other hand, the pulsation dampening devices described in the Sharp, et al patent suggest the provision of side branch inlet and outlet conduits with respect to a pressure vessel type acoustic capacitance and which conduits merely open into the sidewall of the vessel.
In many fluid transmission systems, pressure vessels and other enlarged volume chambers which are adapted to serve as acoustical capacitances or filter components may, for various reasons, have inlet and outlet conduits arranged according to the teachings of the Damewood, et al patent, that is, extending within the pressure vessel generally along its centerline and opening into the vessel in a direction parallel to or generally aligned with the longitudinal central axis of the vessel. In such arrangements it has been observed that inordinately high pulsation levels occur at frequencies which correspond to the length resonant frequency of the pressure vessel even though the location of the inlet and outlet conduits are as suggested by the above-referenced patents.
In accordance with the present invention it has been discovered that the generation and transmission of pressure pulsations at resonant frequencies may be amplified in an acoustical capacitance of the so-called bottle or pressure vessel type, for example, in those cases wherein the fluid conduits open into the vessel in a direction parallel to or generally aligned with the vessel longitudinal axis. This pulsation amplification or lack or suppression is believed to be due to excitation by velocity coupling or, in other words, amplification of the particle velocity of the fluid in the vessel due to the velocity of a fluid modulating inlet flow. In the cases where vessel resonant frequencies are not involved, one would expect, for example, that absolute particle velocities in the vessel would be less than those in the inlet conduit as a function of the ratio of the diameters of the conduit with respect to the vessel. However, it has been observed that when the frequency of the inlet flow corresponds generally to a length resonant frequency of the pressure vessel that the particle velocities in the vessel can be substantially amplified. Moreover, similar effects occur at the outlet conduit. If the outlet conduit is located at a point of maximum velocity or velocity antinode of an excited resonant frequency standing wave, and if the opening of the outlet conduit is arranged parallel to or generally aligned with the longitudinal axis of the vessel there will be a strong tendency for the flow oscillation in the vessel to modulate flow into the outlet conduit. If the outlet conduit is of a length so as to be resonant at the same frequency then substantial amplification of the modulating flow can occur. In this regard it should be noted that all resonant frequency pulsations of an open ended conduit have a particle velocity maximum or antinode at the open end of the conduit. If this conduit end is located at a point of maximum particle velocity of an acoustic pulsation in the vessel then substantial acoustic intensity can be transmitted out of the vessel in spite of the fact that the conduit junction with the vessel is at a pressure node of a resonant frequency pressure wave. In pursuing the present invention it has been determined that with introduction of flow into and out of an acoustic capacitance such as a so-called bottle or pressure vessel type structure in a specified direction the effects of velocity induced or amplified pressure pulsations can be minimized.